The present disclosure relates to a method and to a separation device for separating a filtrate from a sample fluid, in particular for extracting plasma from whole blood.
Besides centrifuges, which are used mainly in laboratories for separating plasma from whole blood, there have become known a number of devices for obtaining very small amounts of plasma at Point of Care (PoC) settings by separating plasma from whole blood by means of filtering.
In the simplest case plasma separation may be effected by means of a multilayer test strip as described in U.S. Pat. No. 5,262,067 A (BOEHRINGER MANNHEIM), where a transport layer on an inert carrier layer is provided for transporting sample fluid (whole blood) from an input area to a measuring area. The transport layer may for instance be made of glass fibre mat, which in the input area is covered by a plasma separation layer. The procedure is however only suitable for analysers which process test strips.
From WHATMAN INC. Florham Park, N.J. 07932, USA, there is known a separation device under the name of “Mini-UniPrep”, which is suitable for preparing samples for High Performance Liquid Chromatography (HPLC). The unfiltered sample is filled into a sample container and then a filtration plunger is introduced, which has a filter at its front end. The filtration plunger is pushed into the sample container until the separated filtrate fills its interior while the replaced air is vented through a venting opening. The separation device may thereafter be directly inserted into the sample changer unit of an analyser. Withdrawal of the filtrate may be carried out via a septum in the cap of the filtration plunger. It is a disadvantage of this known separation device that the pressure exerted on the filter in the filtration plunger cannot be applied in an reproducible and uniform manner and that pressure peaks that are detrimental to the sample cannot be avoided. This is particularly harmful if the device is used for separating plasma from whole blood since pressure peaks may cause bursting of red blood cells (RBCs) (haemolysis), leading to undesirable contamination of the plasma fraction by the released content substances of the RBCs.
From U.S. Pat. No. 4,990,253 A, especially from FIGS. 4 to 6, a fluid sampling filtration device is known. An outer container of the device filled with a sample to be filtered slidably receives a hollow plunger having filter media disposed near a front end and sealing means disposed in an annular groove about the periphery of the plunger. In use, a liquid sample to be filtered is placed in the outer container. The plunger is inserted filter end first into the open end of the outer container and the sealing means sealingly engages the inner wall to form an air-tight seal between the outer container and the plunger. As the plunger is depressed further into the outer container, air is forced through the filter media and escapes through the loosely fitting cap. Once the plunger reaches the surface of the sample fluid to be filtered, a fixed quantity of air is trapped between the sealing means (O-ring) and the fluid level and, upon further depression of the plunger this trapped air is compressed. The pressurized air in turn forces the fluid sample through the filter media and into a collecting chamber in the interior of the hollow plunger. Filtration is complete when the plunger hits the bottom end of the outer container. Simultaneously, the O-ring snaps past nubs formed in the inner wall of container to lock the two components together. It is a disadvantage of this known filtration that the filtration is terminated after the plunger contacts the bottom of the outer container.
EP 0 297 441 A2 discloses a separation and transfer device comprising a container tube for holding a desired quantity of a liquid and an open-ended, tubular plunger having an O-ring for forming a liquid-tight seal with the interior of the container tube, wherein the seal is maintained while the plunger slides within the tube. The device further comprises a liquid collection cup which is positioned below the plunger while the plunger is depressed. The collection cup and the plunger are furnished with means for allowing the passage of displaced gas during the depression of the plunger. The separation is completed when the plunger hits the bottom of the container tube. There are the same disadvantages as stated above.
It is an object of the present disclosure to propose improvements of the separation device as described above (e.g., “Mini-UniPrep” by WHATMAN INC. or U.S. Pat. No. 4,990,253 A), which will permit the reproducible extraction of plasma samples from relatively small whole blood samples whilst providing easy handling for the user.